This application relates to hard disc drives and more particularly to an apparatus and method for adjusting a velocity profile of an actuator arm to the actual acceleration capabilities of the system.
The storage medium for a disc drive is a flat, circular disc capable of storing data as localized magnetic fields. The data, that are stored upon the disc, find physical representation through these localized magnetic fields. The data are recorded on a disc in concentric, circular paths known as tracks.
The localized magnetic fields can be detected by a head when the field is brought in close proximity to the head. During operation the disc continually rotates, meaning that for each rotation, a head fixed a given radius from the center of the disc would read the data recorded in a given track. An actuator arm swings the head in an arc across the disc surface to allow the head to read or write data along a different track.
The read/write head is mounted upon the distal end of the actuator arm, and the arm is moved by a servo control system. Accordingly, the track position of the head is controlled by the servo system. When the head needs to access a different track, the actuator arm swings the head to the desired track location. The motion of the head from one track to another includes an acceleration and a deceleration phase, and the period during which head movement occurs is known as the seek time. For drive performance, it is desirable to minimize the seek time.
In a conventional disc drive, the movement of the actuator arm is controlled by a feedback control loop, and may include feedforward control as well. The control process typically works as follows. A ROM (a ROM memory device is a memory device which stores data that either cannot be erased or cannot be erased during normal operation) look-up table possesses a worst case velocity profile that indicates the target velocity of the head, given the head""s distance from the desired track. Such a table assumes a worst case rate of deceleration. Typically, the table yields a target velocity for a given distance parameter based upon the relationship v=[2ax]1/2, where v represents the target velocity, a represents the worst case acceleration, and x represents the distance that the head must travel, along an arc centered about the arm""s pivot point, to reach its desired track position. When movement begins, the arm is accelerated with the maximum torque possible. At intervals, the control system gathers information regarding the actual velocity of the head, and the head""s distance from the desired track position. Using the distance measurement, the ROM table is accessed to retrieve a target velocity for the arm and thus the head. Next, the difference between the target velocity and the actual velocity of the head is found. Acceleration continues until the actual velocity of the head meets the target velocity or a predetermined maximum velocity, whichever is lower. When the actual velocity exceeds the target velocity, deceleration commences.
During deceleration, the control system once again periodically gathers information regarding the actual velocity of the head, and the head""s distance (again, measured along an arc centered about the arm""s pivot) from the desired track position. Using the distance measurement, the ROM table is accessed to retrieve the target velocity of the head. Next, the difference between the target velocity and the actual velocity of the head is found. The servo system is fed with a current that is proportional to the difference between the head""s actual and target velocity, and a resulting torque will be applied to the actuator arm, decelerating the arm. Deceleration continues until the head comes to rest at the desired track position.
Importantly, under this conventional scheme deceleration is controlled to follow a worst case deceleration profile, which is calculated based upon a presumed deceleration ability of the system, given worst case mechanical and environmental factors. By using the worst case profile, it is ensured that even in the worst case scenario, a head will not overshoot its desired target during a seek operation. Assuming the system were actually able to decelerate at rates greater than the assumed nominal rate, the system would possess the ability to transport the head a greater distance before decelerating and then decelerate at a greater rate, thereby reducing the seek time.
U.S. Pat. No. 4,899,234 (xe2x80x9cthe ""234 patentxe2x80x9d) describes one scheme by which a control system can dynamically adapt to the deceleration capacity of the servomechanism it is controlling. The ""234 patent teaches a scheme wherein a disc drive is preloaded with a time value representative of the time consumed, under worst case operating conditions, for the drive""s head to travel the number of tracks required for the head to reach maximum velocity under best case operating conditions. At the commencement of each seek operation, the actual time required for the head to travel the number of tracks required to reach maximum velocity under best case operating conditions is measured, assuming the seek operation requires the head to traverse at least that many tracks. The worst case time is then divided by the measured time, producing a performance ratio.
During a seek operation, the control system described in the ""234 patent functions as described in the conventional case, with the following exceptions: the velocity profile is designed assuming a worst case acceleration capacity instead of a nominal acceleration capacity, and the target velocity returned from the velocity profile is multiplied by the performance ratio. In concert, these alterations allow the control system of the ""234 patent to dynamically adapt to the deceleration capacity of the servomechanism it is controlling.
Certain factors influence the efficacy of the control system of the ""234 patent. One such factor is the precision with which the drive is capable of measuring time. Because the performance ratio described in the ""234 patent is calculated from a measured time interval and the performance ratio is used to scale the velocity profile, it is essential that the system be capable of measuring time precisely. A system with significant quantization error with respect to time will propagate that error, yielding a velocity profile which has been inaccurately scaled. Another factor affecting the efficacy of the control system of the ""234 patent is that the system requires a seek operation traversing a certain number of tracksxe2x80x94the number of tracks required for the head to reach maximum velocity under best case operating conditionsxe2x80x94before a performance ratio can be calculated and the system adapted to the deceleration capacity of the servomechanism.
The method and apparatus in accordance with the present invention solves the aforementioned problem and other problems by adapting the nominal velocity profile to the actual deceleration capabilities of the system. The method involves controlling the swing of an actuator arm with feedback and feedforward control systems. The method commences by accelerating the actuator arm with maximum torque, and obtaining the actual acceleration of the actuator arm. A performance ratio is then calculated as the ratio between the actual acceleration and the nominal acceleration of the actuator arm. Acceleration may be obtained by measuring the distance the head travels over a given interval of time. This permits the system to determine acceleration, and therefore the performance ratio, by measuring head displacement, a variable it can determine with precision. Further, acceleration may be measured prior to the head achieving maximum velocity, meaning this method is effective even for relatively small seek lengths. The performance ratio is used to scale the distance axis of the nominal velocity profile by multiplying the distance parameter by the performance ratio before indexing into the velocity profile to retrieve a target velocity. The performance ratio is also used to scale a feedforward deceleration control signal before it is fed forward into the control loop.
This method reduces the seek time of the non-worst case disc drive by allowing the control system to take advantage of the full deceleration capacity of the system. This is characterized by the head being transported a greater distance before the commencement of deceleration, coupled with deceleration occurring at greater rates. However, since the profile is adaptive, it allows the worst case drive to operate according to worst case deceleration assumptions, thereby not overshooting its target during a seek operation.
The apparatus includes a servomechanism, which is used to apply torque to an actuator arm. A transducer is coupled to the servomechanism so that it produces a signal representative of the position of the head. A microprocessor is operably connected to the transducer and to a ROM possessing a velocity profile. The microprocessor calculates the actual velocity of the actuator arm from the position signal, and utilizes the position signal scaled by a performance ratio to access the ROM table for a target velocity. Then, the microprocessor subtracts the actual velocity from the target velocity to produce an error quantity, multiplies the error quantity by a constant to produce an error product, and adds the error product to a ratio-scaled feedforward signal. The microprocessor then converts the aforementioned sum into an analog signal, which a power amplifier receives, and magnifies so as to drive the servomechanism.
These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.